The research focus of the Unit of Molecular Virology in the Laboratory of Cell and Molecular Regulation has been the determination of the individual steps in the process of enveloped retroviral entry. The first step of retroviral entry requires virus binding to a cell surface receptor followed by the second, or fusion step of the retroviral envelope membrane and the cytoplasmic cell membrane. The culminating step is the release of the nucleocapsid into the cytoplasm. We use Gibbon Ape Leukemia Virus (GALV) as a model to study enveloped virus entry because GALV is a simple virus and lacks the accessory proteins which are common features of more complex enveloped viruses including vaccinia, herpes and HIV. The second reason for using GALV as a model for virus entry studies is that we have developed GALV-based retroviral vectors, enabling us to use vectors instead of Replication-competent GALV in our studies. Finally, the human cDNA encoding the GALV receptor has been cloned. We have determined that 1) the GALV receptor normally functions as a Type III NaPi cotransporter, designatedPit1.The distinguishing feature of Type III Pi transporters such as Pit1 is that they are ubiquitously expressed and appear to function in a housekeeping role absorbing Pi from interstitial fluid for normal cellular functions such as cellular metabolism, signal transduction, nucleic acid and lipid synthesis; Type I and II transporters are exclusively expressed in the kidney. We have also determined that Pit- mediated Pi uptake is specifically blocked by GALV infection and is regulated differently when compared to the other Type III Pi transporter, Pit2. 2) There are three discrete regions within Pit1 that interactively facilitate GALV entry. 3) Expression of functional Pit1on the cell surface is necessary but not sufficient for GALV entry. Finally, we have developed and tested retroviral vectors capable of efficiently delivering and stably expressing genes in chick pineal cells and in rat neuronal progenitor cells. - C-Type, GALV, Retrovirus, virus binding, fusion, Type III Pi transporters, retroviral vectors